1. Field of the Invention
The present invention relates to a power supply circuit for use with, for example, signal lights utilizing LEDs (light-emitting diodes).
2. Description of the Related Art
An attempt has been made to use signal lights utilizing LEDs as traffic lights. LEDs can provide various colors and can be used as excellent light sources for lighting equipment, because LEDs have higher luminous efficiencies than do incandescent lamps and are excellent in terms of energy conservation.
In order to use an LED light source as a light source of a traffic light or ordinary lighting equipment indoor and outdoor, while guaranteeing safety and long service life, the LED light source must be used in consideration of great changes in environmental temperature from winter to summer.
For example, when a signal light utilizing an LED is used at temperatures lower than 0.degree. C. in winter, heat is rapidly generated at a light-emitting portion of the LED when the LED is turned on, with the result that an internal material constituting the LED expands thermally. Since the entire structure of the LED is in a contracted state at such low temperature, the internal expansion generated when the LED is turned on generates mechanical stress within the LED structure.
Accordingly, repeated turning on and off operations create severe operating conditions, possibly resulting in breakage of the LED, depending on the conditions.
Therefore, in order to secure a required quantity of light and mitigate the above-described phenomena, current supplied to the LED must be determined in consideration of the temperature characteristics of the LED such that the current becomes suitable for temperatures encountered during winter.
Further, when an LED is used at high temperature in summer, the following points must be taken into consideration. A manufacturer of LEDs provides various ratings in relation to use of LEDs. According to such ratings, when an LED is used in an environment having a temperature higher than room temperature, the maximum current permitted to be supplied to the LED is lower than that at room temperature.
That is, when during actual use an LED is placed in a high-temperature atmosphere having a temperature higher than about 40.degree. C., the current supplied to the LED must be reduced as compared with that supplied at ordinary temperature, in order to prevent thermal damage such as thermal fusion of an internal material that constitutes the LED.
Therefore, for such high temperature use, the supply current must be determined so as to secure a required quantity of light and not to exceed a rated current specified for high temperatures encountered during summer.
Further, conventional signal lights utilizing an LED have a drawback in that there arises a time delay between supply of power and actual emission of light from the LED.